1. Field of the Invention
The present invention relates generally to direct current power units, and more particularly, to a direct current power unit which is employed for supplying a bias voltage for development and can switch the polarity of an output voltage into negative or positive polarity, in an electrostatic recording apparatus such as an electrostatic copying apparatus and an electrostatic printer.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing an example of a main portion of an electrostatic recording apparatus, and FIG. 2 is a diagram showing the principle of a direct current power unit for supplying a bias voltage for development.
As shown in FIG. 1, in the electrostatic recording apparatus, positive charges are applied to the surface of a photoreceptor drum 2 by a charger 4 and then, exposure light 6 reflected from an image is irradiated on the surface of the photoreceptor drum 2 so that a latent electrostatic image is formed. A toner is then adhered to the surface of the photoreceptor drum 2 by a brush 8 to which a bias voltage for development is applied. Since such an electrostatic recording apparatus has been conventionally known, the description of the subsequent processes is omitted.
In the above described electrostatic recording apparatus, if and when black and white of an image to be recorded are inverted to each other, the polarity of the bias voltage for development applied to the brush 8 must be inverted. As a direct current power circuit for supplying such a bias voltage for development, a circuit as shown in FIG. 2 is employed.
In FIG. 2, when it is desired to obtain a positive bias voltage for development, a negative power supply 10b is turned off and one end of a resistance 12b is grounded, so that a positive output voltage +V.sub.0 is obtained at an output terminal 14 from a positive power supply 10a through a resistance 12a. Contrary to this, when it is desired to obtain a negative bias voltage for development, the positive power supply 10a is turned off and one end of the resistance 12a is grounded, so that a negative output voltage -V.sub.0 is obtained at the output terminal 14 from the negative power supply 10b through the resistance 12b. The positive or negative output voltage V.sub.0 outputted to the output terminal 14 is supplied to the above described brush 8 as a bias voltage for development.
FIG. 3 is an electric circuit diagram of a direct current power circuit structured using a transformer.
In FIG. 3, one end of a secondary winding 18 of a transformer 16 is grounded. The secondary winding 18 has another end connected to the anode of a rectifier diode 20a as well as the cathode of a rectifier diode 20b. The cathode of the rectifier diode 20a is connected to one end of a smoothing capacitor 17a as well as a voltage regulator circuit 22a. The anode of the rectifier diode 20b is connected to one end of a smoothing capacitor 17b as well as a voltage regulator circuit 22b. The smoothing capacitors 17a and 17b have respective other ends connected to ground. The voltage regulator circuits 22a and 22b are used for making a d-c (direct current) voltage a constant voltage.
Output voltages from the voltage regulator circuits 22a and 22b are outputted to an output terminal 14 through resistances 12a and 12b, respectively. The resistances 12a and 12b are used for turning off either one of the voltage regulator circuits 22a and 22b as well as dividing the output voltage of the other voltage regulator circuit when the output terminal is grounded. More specifically, when the voltage regulator circuit 22b is turned off and the output terminal side thereof is grounded, a positive voltage outputted from the voltage regulator circuit 22a is divided by the resistances 12a and 12b, so that the positive output voltage +V.sub.0 is outputted to the output terminal 14. Contrary to this, when the voltage regulator circuit 22a is turned off and the output terminal side thereof is grounded, a negative voltage outputted from the voltage regulator circuit 22b is divided by the resistances 12b and 12a, so that the negative output voltage -V.sub.0 is obtained at the output terminal 14.
However, in the direct current power circuit as shown in FIG. 3, the output voltage of the voltage regulator circuit 22a or 22b is divided into approximately halves by the resistances 12a and 12b, to obtain the positive or negative output voltage V.sub.0, so that a high voltage which is two or more times a desired output voltage V.sub.0 is required as the output voltages of the voltage regulator circuits 22a and 22b. Therefore, large capacity circuit parts are required as circuit parts such as the transformer 16, so that each of the circuit parts is increased in size. Accordingly, the entire power unit is increased in size, so that the cost thereof becomes higher.
Additionally, in the power circuit shown in FIG. 3, either one of the voltage regulator circuits 22a and 22b must be turned off to ground the output terminal end thereof in order to obtain either one of the output voltages, so that circuit structure therefor becomes complicated.